WGS84 Lat/Long to Grid Reference Conversion
This page will convert latitude and longitude (assuming WGS84 datum) into rectilinear coordinates suitable for use on this site (handles both Great Britain and Irish grids).
See also nearby.org.uk which offers a similar converter.
Note: The individual deg/min/sec boxes all take decimal values too. So a value like '56:34.2345' (degrees with decimal minutes), can enter '34.2345' in the min box.